A known device disclosed, for instance, in Patent Document 1 exercises active control to forcibly switch the air-fuel ratio prevailing upstream of a catalyst between a lean air-fuel ratio and rich air-fuel ratio to measure the amount of oxygen storage by the catalyst. This device detects catalyst deterioration in accordance with the measured oxygen storage amount.
Patent Document 1: JP-A-2004-176611
Patent Document 2: Japanese Patent No. 2812023
Patent Document 3: Japanese Patent No. 3264234
Patent Document 4: Japanese Patent No. 2806248